prost = [2]
n=10000
for i in range(3,n+1):
    ksun = 0
    for j in prost:
        if i %j==0:
            ksun=1
    if ksun==0:
        prost.append((i))

a = input()
b=[]
if int(a) in prost:
    b.append
b.extend([a[:-1],a[1:],a[2:],a[1:-1],a[0],a[1],a[2],a[3]])

x=[]
for i in range(len(b)):
    s=b[i]
    for j in range(len(b)):
        if int(s)==int(prost[j]):
            x.append(int(s))
if len(x)>0:
    print(max(x))
else:
    print(0)
